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You may be a buy-and-hold investor who's content with your portfolio. But if a spike in the price of one of your holdings suddenly presents you with a big gain, should you take the money and run?

Shareholders in companies such as WSB Holdings (ticker: WSB) understand the dilemma. The Washington state savings bank popped 83% to $5.34 in one day in September. Ditto for investors in
magicJack VocalTeccall 3.4482758620689653%magicJack VocalTec Ltd.U.S.: NasdaqUSD7.5
0.253.4482758620689653%
/Date(1481320800402-0600)/
Volume (Delayed 15m)
:
85887
P/E Ratio
13.392857142857142Market Cap
114948746.681213
Dividend Yield
N/ARev. per Employee
1067420More quote details and news »callinYour ValueYour ChangeShort position
(CALL), which makes multimedia and voice-over-Internet protocol for communications outfits and leapt 13% in a day, or
Lynas
(LYSCF), a mining company that rose 49% in a session.

Lynas shares bounced when the company got government approval to start a Malaysian rare-earth processing plant, and magicJack had just lifted its earnings forecast. But even when the catalyst seems clear, its effect is sometimes counterintuitive. WSB shares, for instance, took off after it acquired another bank, a move that usually causes a stock to fall.

When in doubt, investors should turn to that collection of numbers and ratios that describes valuation, says Shawn Carpenter, CEO of stock researcher YCharts (ycharts.com). The primary question is whether the new share price is temporarily above or below what a reasonable buyer would pay for the company's future earnings prospects, otherwise known as its fair-market value. If too high, the stock might be poised to fall back toward the mean, or the average, of its fair-market value -- even though it may be a good long-term hold.

One giveaway is an out-of-whack price/earnings ratio. WSB's 83% move left it with a P/E of 39, almost three times that of the Standard & Poor's 500. MagicJack's P/E was a heady 32. But momentum stocks often sport high P/Es for months on end. No single metric tells the whole story, says Carpenter, so YCharts rolls several measures into an overall Pro Value Score. Subscribers can get ratings on 4,700 U.S.-listed companies for $50 monthly.

Also considered are price-to-sales ratio, book value and cash-flow yield -- not all of which will be relevant for every ticker. An important one: WSB's earnings yield was a low 2.6% and magicJack's was only 2.3% -- little better than a Treasury bill. But magicJack's net income had almost tripled over the past 12 months, while WSB's had been cut in half. Different long-term prospects, but YCharts judged both overvalued in the short term.

CAPITALCUBE (capitalcube.com) approaches that analysis from a slightly different angle, emphasizing how a company stacks up against its peers. For $79 monthly, it rates no less than 43,000 issues, and is a place to find ratings on stocks that trade offshore, like Australia-based Lynas.

The terminology of CapitalCube sounds different -- relative share-price performance, earnings leverage, and sustainability of returns—but the same areas are covered. CapitalCube puts magicJack's prospects in the upper third of its peer group, but assigns WSB to the lower quartile: "WSB's relatively low net margins and poor capital turns suggest a problematic operating strategy," according to the CapitalCube site. It would be understandable if someone with gains on either stock took their money off the table.

THEN THERE ISAlpha Natural Resources
(ANR). After a yearlong decline, its shares bounced 12% off a $5.28 bottom one day in September, then kept ticking up over the next seven trading days to hit $9.05 intraday before pulling back. Those who sold that first day would have missed a big part of a 71% gain.

A more fine-grained timing tool, a price/volume chart from a site like StockCharts.com, could have helped. For monthly fees starting at $15, subscribers can access many chart styles that illustrate the tug-of-war between share buyers and sellers along with technical signals that pinpoint who's winning. One popular signal, the Relative Strength Index, showed that Alpha was "oversold" before bouncing and "overbought" at its price peak.

Share prices often swing far above, then far below, fair value, tracing a roller-coaster pattern over time. Of course you're supposed to buy at the trough and sell at the top. These sites can help you recognize them.